JPEG decoding

Figure 45.12 Compression ratio control in JPEG is effected by altering the quantizer matrix: The larger the entries in the quantizer matrix, the higher the compression ratio. The higher the compression ratio, the higher the reconstruction error. At some point, compression artifacts will become visible.

DCT-1 Q-1 VLE-1

In ISO JPEG, the quantizer matrix is directly conveyed in the bitstream. In the DV adaptation of JPEG, several quantizer matrices are defined in the standard; the bitstream indicates which one to use.

Compression ratio control

The larger the entries in the quantizer matrix, the higher the compression ratio. Compression ratio control in JPEG can be achieved by altering the quantizer matrix, as suggested by the manual control sketched in

Figure 45.12. Larger step sizes give higher compression ratios, but image quality is liable to suffer if the step sizes get too big. Smaller step sizes give better quality, at the expense of poorer compression ratio. There is no easy way to predict, in advance of actually performing the compression, how many bytes of compressed data will result from a particular image.

The quantizer matrix could, in principle, be chosen adaptively to maximize the performance for a particular image. However, this isn’t practical. JPEG encoders for still images generally offer a choice of several compression settings, each associated with a fixed quantizer that is chosen by the system designer.

Because different quantizer matrices may be associated with different images, the quantizer matrix must be conveyed to the decoder, as sketched in

Figure 45.13, either as part of the file, or through a side channel. In colour images, separate quantizers are typically used for the luma and chroma components. In stillframe applications, the overhead of this operation is small. In a realtime system, the overhead of conveying quantizer matrices with every frame, or even within

a frame, is a burden.

A modified approach to compression ratio control is adopted in many forms of M-JPEG (and also, as you will see in the next chapter, in MPEG): Reference luma and chroma quantizer matrices are established, and all of

Figure 45.13 Because the quantizer is adjustable, the quantizer matrix must be conveyed through a side channel to the decoder. In colour images, separate quantizers are used for the luma and chroma components.

DCT-1 Q-1 VLE-1

TABLE

The notation mquant is found in the ITU-T H.261 standard for teleconferencing; mquant (or MQUANT) is not found in JPEG or MPEG documents, but is used informally.

Hamilton, Eric (1992), JPEG File Interchange Format, Version 1.02 (Milpitas, Calif.: C-Cube Microsystems). This informal document was endorsed by ECMA and was published in June 2009 as ECMA TR/98 having the same title.

their entries are scaled up and down by a single numerical parameter, the quantizer scale factor (QSF, sometimes denoted Mquant). QSF can be varied to accomplish rate control.

As mentioned earlier, JPEG ordinarily uses luma/chroma coding with 4:2:0 chroma subsampling. However, the JPEG standard accommodates R’G’B’ image data without subsampling, and also accommodates four-channel image data (such as CMYK, used in print) without subsampling. These schemes are inapplicable to video.

JPEG/JFIF

The ISO/IEC standard for JPEG defines a bitstream, consistent with the original expectation that JPEG would be used across communication links. To apply the JPEG technique to computer files, a small amount of supplementary information is required; in addition, it is necessary to encode the ISO/IEC bitstream into

a bytestream. The de facto standard for single-image JPEG files is the JPEG File Interchange Format (JFIF), adopted by an industry group led by C-Cube. A JFIF file encapsulates a JPEG bitstream, along with a small amount of supplementary data. If you are presented with an image data file described as JPEG, it is almost certainly an ISO/IEC JPEG bitstream in a JFIF wrapper.

The JPEG standard itself implies that JPEG could be applied to linear-light RGB data. However, JPEG has poor visual performance unless applied to perceptually coded image data, that is, to gamma-corrected R’G’B’.